1. Field of the Invention
The present invention relates generally to a data transmission/reception apparatus and method in a CDMA (Code Division Multiple Access) mobile communication system, and in particular, to an apparatus and method for transmitting/receiving data using a variable modulation mode at retransmission.
2. Description of the Related Art
Currently, the mobile communication system has evolved from an early voice-based communication system into a high-speed, high-quality radio data packet communication system for providing a data service and a multimedia service. In addition, a 3rd generation mobile communication system, divided into an asynchronous 3GPP (3rd Generation Partnership Project) system and a synchronous 3GPP2 (3rd Generation Partnership Project 2) system, is being standardized for a high-speed, high quality radio data packet service. For example, standardization on HSDPA (High-Speed Downlink Packet Access) is performed by the 3GPP, while standardization on 1xEV-DV is performed by the 3GPP2. Such standardizations are carried out to find out a solution for a high-speed, high-quality radio data packet transmission service of 2 Mbps or over in the 3rd generation mobile communication system. A 4th generation mobile communication system is proposed to provide a high-speed, high-quality multimedia service superior to that of the 3rd generation mobile communication system.
A principal factor of impeding the high-speed, high-quality radio data service lies in the radio channel environment. The radio channel environment is frequently changed due to a variation in signal power caused by white nose and fading, shadowing, Doppler effect caused by a movement of and a frequent change in speed of a UE (User Equipment), and interference caused by other users and a multipath signal. Therefore, in order to provide the high-speed radio data packet service, there is a need for an improved technology capable of increasing adaptability to the variation in the channel environment in addition to the general technology provided for the existing 2nd or 3rd generation mobile communication system. A high-speed power control method used in the existing system also increases adaptability to the variation in the channel environment. However, both the 3GPP and the 3GPP2, carrying out standardization on the high-speed data packet transmission, make reference to AMCS (Adaptive Modulation/Coding Scheme) and HARQ (Hybrid Automatic Repeat Request).
The AMCS is a technique for adaptively changing a modulation mode and a coding rate of a channel encoder according to a variation in the downlink channel environment. Commonly, to detect the downlink channel environment, a UE measures a signal-to-noise ratio (SNR) and transmits the SNR information to a Node B over an uplink. The Node B predicts the downlink channel environment based on the SNR information, and designates proper modulation mode and coding rate according to the predicted value. The modulations available for the AMCS include QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary Phase Shift Keying), 16 QAM (16-ary Quadrature Amplitude Modulation) and 64 QAM (64-ary Quadrature Amplitude Modulation), and the coding rates available for the AMCS include 1/2 and 3/4. Therefore, an AMCS system applies the high-order modulations (16 QAM and 64 QAM) and the high coding rate 3/4 to the UE located in the vicinity of the Node B, having a good channel environment, and applies the low-order modulations (QPSK and 8 PSK) and the low coding rate 1/2 to the UE located in a cell boundary. In addition, compared with the existing high-speed power control method, the AMCS decreases an interference signal, thereby improving the average system performance.
The HARQ is a link control technique for correcting an error by retransmitting the transmitted data upon occurrence of a packet error at initial transmission. Generally, the HARQ is classified into Chase Combining (CC), Full Incremental Redundancy (FIR), and Partial Incremental Redundancy (PIR).
The CC is a technique for transmitting a packet such that the whole packet transmitted at retransmission is equal to the packet transmitted at initial transmission. In this technique, a receiver combines the retransmitted packet with the initially transmitted packet previously stored in a buffer thereof by a predetermined method. By doing so, it is possible to increase reliability of coded bits input to a decoder, thus resulting in an increase in the system performance. Combining the two same packets is similar to repeated coding in terms of effects, so it is possible to increase a performance gain by about 3 dB on the average.
The FIR is a technique for transmitting a packet comprised of only the redundant bits generated from the channel encoder instead of the same packet, thus to improve performance of a decoder in the receiver. That is, the FIR uses the new redundant bits as well as the initially transmitted information resulting in a decrease in the coding rate, thereby improving performance of the decoder. It is well known in coding theory that a performance gain by a low coding rate is higher than a performance gain by repeated coding. Therefore, the FIR is superior to the CC in terms of only the coding gain.
Unlike the FIR, the PIR is a technique for transmitting a combined data packet of the information bits and the new redundant bits at retransmission. Therefore, the PIR can obtain the similar effect to the CC by combining the retransmitted information bits with the initially transmitted information bits during decoding, and also obtain the similar effect to the FIR by performing the decoding using the redundant bits. The PIR has a coding rate slightly higher than that of the FIR, showing average performance between the FIR and the CC. However, the HARQ should be considered in the light of not only the performance but also the system complexity, such as a buffer size and signaling of the receiver, so it is not easy to determine only one of them.
The AMCS and the HARQ are separate techniques for increasing adaptability to the variation in the link environment. Preferably, it is possible to remarkably improve the system performance by combining the two techniques. That is, the transmitter determines a modulation mode and a coding rate that is proper for a downlink channel condition by the AMCS, and then transmits packet data according to the determined modulation mode and coding rate, and the receiver sends a retransmission request upon failure to decode the data packet transmitted by the transmitter. Upon receipt of the retransmission request from the receiver, the Node B retransmits the data packet by the HARQ.
FIG. 1 illustrates an existing transmitter for high-speed packet data transmission, wherein it is possible to realize various AMCSs and HARQs by controlling a channel encoder 112. Referring to FIG. 1, the channel encoder 112 is comprised of an encoder and a puncturer. When input data that is proper to a data rate is applied to an input terminal of the channel encoder 112, the encoder performs encoding in order to decrease a transmission error rate. Further, the puncturer punctures an output of the encoder according to a coding rate and an HARQ type previously determined by a controller 120, and provides its output to a channel interleaver 114. Since the future mobile communication system needs a powerful channel coding technique in order to reliably transmit high-speed multimedia data, the channel encoder of FIG. 1 is realized by a turbo encoder 200 with a mother coding rate R=⅙ and a puncturer 216, as illustrated in FIG. 2. It is known that channel coding by the turbo encoder 200 shows performance closest to the Shannon limit in terms of a bit error rate (BER) even at a low SNR. The channel coding by the turbo encoder 200 is also adopted for the HSDPA and 1xEV-DV standardization by the 3GPP and the 3GPP2. The output of the turbo encoder 200 can be divided into systematic bits and parity bits. The “systematic bits” refer to actual information bits to be transmitted, while the “parity bits” refer to bits used to help a receiver correct a possible transmission error. The puncturer 216 selectively punctures the systematic bits or the parity bits output from the encoder 200, thus to satisfy a determined coding rate.
Referring to FIG. 2, upon receiving one transmission frame, the channel encoder outputs the intact transmission frame as a systematic bit frame X. The transmission frame is also provided to a first channel encoder 210, and the first channel encoder 210 performs coding on the transmission frame and outputs two different parity bit frames Y1 and Y2. In addition, the transmission frame is also provided to an interleaver 212, and the interleaver 212 interleaves the transmission frame. The intact interleaved transmission frame is transmitted as an interleaved systematic bit frame X′. The interleaved transmission frame is provided to a second channel encoder 214, and the second channel encoder 214 performs coding on the interleaved transmission frame and outputs two different parity bit frames Z1 and Z2. The systematic bit frames X and X′ and the parity bit frames Y1, Y2, Z1 and Z2 are provided to the puncturer 216 in a transmission unit of 1, 2, . . . , N. The puncturer 216 determines a puncturing pattern according to a control signal provided from the controller 120 of FIG. 1, and performs puncturing on the systematic bit frame X, the interleaved systematic bit frame X′, and the four different parity bit frames Y1, Y2, Z1 and Z2 using the determined puncturing pattern, thus outputting desired systematic bits S and parity bits P.
As described above, the puncturing pattern used to puncture the coded bits by the puncturer 216 depends upon the coding rate and the HARQ type. That is, in the case of the CC, it is possible to transmit the same packet at each transmission by puncturing the coded bits such that the puncturer 216 has a fixed combination of the systematic bits and the parity bits according to a given coding rate. For the FIR and PIR, the puncturer 216 punctures the coded bits in a combination of the systematic bits and the parity bits according to the given coding rate at initial transmission, and punctures the coded symbols in a combination of various parity bits at each retransmission, thus resulting in a decrease in the overall coding rate. For example, in the case of the CC with the coding rate 1/2, the puncturer 216 can continuously output the same bits X and Y1 for one input bit at initial transmission and retransmission, by fixedly using [1 1 0 0 0 0] in the order of the coded bits [X Y1 Y2 X′ Z1 Z2] as the puncturing pattern. In the case of the IR, the puncturer 216 outputs the coded bits in the order of [X1 Y11 X2 Z21] at initial transmission and in the order of [Y21 Z21 Y12 Z12] at retransmission for two input bits, by using [1 1 0 0 0 0; 1 0 0 0 0 1] and [0 0 1 0 0 1; 0 1 0 0 1 0] as the puncturing patterns at initial transmission and retransmission, respectively. Meanwhile, though not separately illustrated, an R=⅓ turbo encoder adopted by the 3GPP2 can be realized by the first channel encoder 210 and the puncturer 216 of FIG. 2.
A packet data transmission operation by the AMCS system and the HARQ system realized by FIG. 1 will be described herein below. Before transmission of a new packet, the controller 120 of the transmitter determines a proper modulation mode and data rate based on the downlink channel condition information provided from the receiver. The controller 120 provides information on the determined modulation mode and coding rate to the channel encoder 112, a modulator 116 and a frequency spreader 118. A data rate in a physical layer depends upon the determined modulation mode and coding rate. The channel encoder 112 performs bit puncturing according to a given puncturing pattern after performing the encoding by the determined modulation mode and coding rate. The coded bits output from the channel encoder 112 are provided to the channel interleaver 114, in which they are subject to interleaving. The interleaving is a technique for preventing a burst error by randomizing the input bits to disperse data symbols into several places instead of concentrating the data symbols in the same place in a fading environment. For ease of explanation, the size of the channel interleaver 114 is assumed to be larger than or equal to the total number of the coded bits. The modulator 116 symbol-maps the interleaved coded bits according to the determined modulation mode, and outputs modulated symbols. If the modulation mode is represented by M, the number of coded bits constituting one symbol is log2 M. The frequency spreader 118 assigns multiple Walsh codes for transmitting the modulated symbols at the determined data rate, and spreads the modulated symbols with the assigned Walsh codes. When a fixed chip rate and a fixed spreading factor (SF) are used, a rate of symbols transmitted with one Walsh code is constant. Therefore, in order to use the determined data rate, it is necessary to use multiple Walsh codes. For example, when a system using a chip rate of 3.84 Mcps and an SF of 16 chips/symbol uses 16 QAM and a channel coding rate 3/4, a data rate that can be provided with one Walsh code becomes 1.08 Mbps. Therefore, when 10 Walsh codes are used, it is possible to transmit at a data rate of a maximum of 10.8 Mbps.
It is assumed in the transmitter of the high-speed packet transmission system of FIG. 1 that the modulation mode and coding rate determined by the AMCS at initial transmission are used even at retransmission. However, as described before, the high-speed data transmission channel is subject to a change in its channel condition, even in a retransmission period by the HARQ, due to the change in the number of UEs in a cell and the Doppler shift. Therefore, maintaining the modulation mode and the coding rate used at initial transmission results in a reduction in the system performance.
For this reason, the ongoing HSDPA and 1xEV-DV standardizations consider an improved method for changing the modulation mode and the coding rate even in the retransmission period. For example, in a system using the CC as the HARQ, when the HARQ type is changed, a transmitter retransmits a part or the whole of the initially transmitted data packet, and a receiver partially combines the partially retransmitted packet with the whole of the initially transmitted packet, resulting in a reduction in the entire bit error rate of a decoder. Structures of the transmitter and the receiver are illustrated in FIGS. 3 and 4, respectively.
As illustrated in FIG. 3, the transmitter for the improved method further includes a partial Chase encoder 316 in addition to the transmitter of FIG. 1. Referring to FIG. 3, coded bits generated by encoding input data according to a given modulation mode and coding rate by a channel encoder 312 are provided to the partial chase encoder 316 after being interleaved by an interleaver 314. The partial Chase encoder 316 controls an amount of data (or the number of data bits) to be transmitted at retransmission among the interleaved coded bits based on information on a modulation mode used at initial transmission, a current modulation mode, and the number of Walsh codes to be used, provided from the controller 322. A modulator 318 performs symbol-mapping on the coded bits output from the partial Chase encoder 316 according to a given modulation mode, and provides its output to a spreader 320. The spreader 320 assigns a needed number of Walsh codes among the available Walsh codes, and frequency-spreads the data symbols received from the modulator 318 with the assigned Walsh codes. Here, the channel coding rate at retransmission is identical to the channel coding rate at initial transmission, and the number of the Walsh codes to be used at retransmission may be different from the number of the Walsh codes used at initial transmission. However, it will be assumed herein that the number of Walsh codes to be used at retransmission is identical to the number of Walsh codes used at initial transmission. Therefore, a symbol rate for the retransmission is identical to a symbol rate for the initial transmission, so it is necessary to adjust the number of the retransmitted coded bits.
FIG. 4 illustrates a structure of a receiver corresponding to the transmitter of FIG. 3. The receiver further includes a partial Chase combiner 416 corresponding to the partial Chase encoder 316 of FIG. 3, in addition to the existing receiver. A despreader 412 despreads the data symbols from the transmitter with the same Walsh codes as used by the transmitter, and provides its output to a demodulator 414. The demodulator 414 demodulates the data symbols from the despreader 412 by a demodulation mode corresponding to the modulation mode used by the transmitter, and outputs a corresponding LLR (Log Likelihood Ratio) value to the partial Chase combiner 416. The LLR value is a value determined by performing soft decision on the demodulated coded bits. The partial Chase combiner 416 substitutes for the soft combiner in the existing receiver. This is because when the modulation used at initial transmission is different from the modulation used at retransmission, the packet combining is partially performed since an amount of the retransmitted data is different from an amount of the initially transmitted data. If the high-order modulation is used at retransmission, the partial Chase combiner 416 performs combining on the whole packet. However, if the low-order modulation is used at retransmission, the partial Chase combiner 416 performs partial combining. A deinterleaver 418 deinterleaves the data from the partial Chase combiner 416 and provides the deinterleaved data to a channel decoder 420. The channel decoder 420 decodes the deinterleaved coded bits into information bits. Though not illustrated in FIG. 4, the receiver performs CRC (Cyclic Redundancy Check) checking on the information bits, and transmits an ACK (Acknowledge) or a NACK (Negative Acknowledge) signal as a confirmation signal to the transmitter according to the CRC checking results, thus to request transmission of new data or retransmission of the errored packet.
FIGS. 5A and 5C illustrate a change in the size of the packet encoded by the partial Chase encoder 316 according to a change in the modulation mode at initial transmission and retransmission.
First, if a modulation rate (or modulation order) at retransmission is lower than a modulation rate at initial transmission, the coded bits output from the partial Chase encoder 316 are less in number at retransmission compared with initial transmission, as illustrated in FIG. 5A. It is assumed in FIG. 5A that 16 QAM is used at initial transmission and QPSK is used at retransmission. Therefore, at retransmission, the transmitter transmits only half of the data packet transmitted at initial transmission.
Next, if a modulation rate at retransmission is higher than a modulation rate at initial transmission, the coded bits output from the partial Chase encoder 316 are larger in number at retransmission compared with initial transmission, as illustrated in FIG. 5C. It is assumed in FIG. 5C that QPSK is used at initial transmission and 16 QAM is used at retransmission. Therefore, at retransmission, the transmitter repeats twice the data packet transmitted at initial transmission.
FIGS. 5B and 5D illustrate a received packet combined by the partial Chase combiner 416 according to a change in the modulation mode at initial transmission and retransmission.
First, if a modulation rate at retransmission is lower than a modulation rate at initial transmission, i.e., if 16 QAM is used at initial transmission and QPSK is used at retransmission, then the receiver additionally receives, at retransmission, only half of the data packet transmitted at initial transmission, as illustrated in FIG. 5B. Therefore, the partial Chase combiner 416 combines the packet received at initial transmission with the half packet received at retransmission, thus to increase reliability of the received signal.
Next, if a modulation rate at retransmission is higher than a modulation rate at initial transmission, i.e., if QPSK is used at initial transmission and 16 QAM is used at retransmission, then the receiver repeatedly receives, at retransmission, the packet transmitted at initial transmission twice, as illustrated in FIG. 5D. Therefore, the partial Chase combiner 416 combines the same packets received three times through initial transmission and retransmission, thus to increase reliability of the received signal.
The high-speed packet transmission system using the CC as the HARQ uses the partial Chase encoder 316 and the partial Chase combiner 416 illustrated in FIGS. 3 and 4, so it can more actively adapt to a change in the channel environment by changing the modulation mode even at retransmission, resulting in an improvement in the system performance. The partial combining on the whole transmission packet contributes to a decrease in the bit error rate, but fails to satisfactorily contribute to a reduction in the frame error rate. This is because the output of the channel interleaver 314 of FIG. 3 is a random combination of the systematic bits and the parity bits from the channel encoder 312. That is, if the packet size at retransmission is smaller than the packet size at initial transmission, the combining cannot be performed on all of the information bits, so the combining effect occurs randomly in a bit unit. In particular, there is a demand for a new method for remarkably reducing a frame error rate by compensating the whole information bits using the feature that the turbo code should be transmitted in combination of the systematic bits and the parity bits even when the system using the CC is required to transmit a smaller packet at retransmission than at initial transmission.